The specific aims of the proposed investigation are all concerned with trace measurements of monoamines released from nerve terminals in the vertebrate brain, especially in the corpus striatum. They are: 1) An experimental evaluation of the electrochemical response of a flow-cell using a pyrolytic graphite electrode, and the determination of the optimum geometry for this electrode in push-pull perfusion applications. This electrode will be used as a continuous monitor of the concentration of electroactive species in the perfusates of rat-brain regions and will allow time resolution of better than one second. The perfusate will be collected and analyzed using LCEC techniques to determine the particular electroactive species which gives rise to the signal. 2) The construction and electrochemical evaluation of micro-voltammetric electrodes from carbon fibers. Multiple arrays of these electrodes will be implanted in discrete regions of the rat brain to measure changes in concentration of electroactive species. The results from part 1) of this section will be used to identify the substances detected by these small electrodes. The tip size of carbon fiber electrodes (5 microns) will permit measurements of neurotransmitters from a very small number of nerve terminals. 3) An investigation of the effects of GABA and acetylcholine on the release of dopamine and serotonin in the corpus striatum of the rat using electrochemical methods of detection. Measurements will be made in synaptosomes, brain slices, and in vivo. A correlation of the experimental results will provide information concerning the relative importance of these modulators on the release of monoamines in this brain region.